Code

private void diagram_SelectionMoving(object sender, ValidationEventArgs e)
{
	MoveLineBehavior b = view.CustomBehavior as MoveLineBehavior;
	if (b == null)
		return;

	PointF current = view.ClientToDoc(view.PointToClient(MousePosition));

	foreach (DiagramLink link in diagram.Selection.Links)
	{
		if (link.Origin is DummyNode && link.Destination is DummyNode)
		{
			for (int i = 0; i < link.ControlPoints.Count; ++i)
			{
				PointF p = link.ControlPoints[i];
				p.X += current.X - startDrag.X;
				p.Y += current.Y - startDrag.Y;
				link.ControlPoints[i] = p;
			}

			link.UpdateFromPoints();
		}
	}

	startDrag = current;
}

private void view_MouseDown(object sender, MouseEventArgs e)
{
	startDrag = view.ClientToDoc(view.PointToClient(MousePosition));
}

internal PointF startDrag;
 

Code

class MoveLineBehavior : LinkShapesBehavior
{
	public MoveLineBehavior(DiagramView view) : base(view)
	{
	}

	public override InteractionState StartDraw(PointF point)
	{
		DiagramLink link = Diagram.GetLinkAt(point, 1);
		if (link != null && link.Origin is DummyNode && link.Destination is DummyNode)
			return new InteractionState(Diagram.Selection, 8, Action.Modify);

		return base.StartDraw(point);
	}
}